FI118025B - Flyback power source and procedure associated with this - Google Patents

Abstract

A method in connection with a Transition Mode Flyback power source, comprising the steps of generating direct voltage as supply voltage for the power source; controlling the switch component of the primary circuit into a conductive state; determining the magnitude of the output voltage; forming a current limit for the current of the primary circuit; determining the magnitude of the primary circuit current of the power source; and controlling the switch component of the primary circuit in turns into a conductive state and a blocking state, depending on the magnitude of the primary circuit current. Forming the current limit for the primary circuit current comprises the step of forming the current limit in response to the magnitudes of the supply voltage and the output voltage in such a way that the current limit is decreased when the supply voltage increases and increased when the supply voltage decreases.

Description

1 1 8025 '1

Flyback power supply and method for this

BACKGROUND OF THE INVENTION The present invention relates to a Flyback-type power supply and a method in connection with such a power supply. In particular, the invention relates to a TM-5 Flyback power supply and a method for providing more accurate power control.

Flyback chippers are commonly used as power sources for low to medium power applications. The Flyback chipper is simple in design and has only a few components in its main circuit. Thus, such a power supply is simple and inexpensive to implement while still being reliable in use.

Figure 1 illustrates in principle a Transition Mode (TM) Flyback power supply with power factor correction circuitry with functions.

an integrated circuit IC1 is connected to drive, 15 components V1 of the power supply of Fig. 1, which are for example a FET switch, are alternately controlled to a conductive and a blocking state. As it passes through the switch and transformer T1, winding N1 passes a current which induces flux to the transformer. Due to the polarities of the transformer windings and the diode D1 connected to the winding N3, the winding N3 does not carry current when the switch V1 is applied. When the switch is directed to the blocking state, the energy charged to the transformer begins to discharge, providing a current to the circuit connected to the windings N3. The output voltage Uout of power -... T source is obtained from capacitor C2 charged through coil N3 · *: *.

....,

In Transition Mode Flyback, the switch component V1 is controlled so that • when the transformer T1 drops to zero, the component is switched to · · * · 25 immediately. In this case, both the operating frequency and the pulse rate change; Ä • · * *. ···. as a function of supply voltage and load. In a standard Flyback connection: • · only the pulse rate is changed so that the peak current remains constant and power can be increased up to the maximum pulse rate limit. Since the frequency of the TM connection also changes, the power limitation does not occur automatically.

Fig. 1 shows rectifier 1 for rectifying an alternating voltage, such as a pre-: ** [: signifies the mains voltage. Typically, the rectifier is formed by a diode bridge, which produces a pulse-like voltage from the AC supply voltage. To compensate for this voltage, a capacitor C1 is coupled further along with a voltage divider coupling 2, which produces a voltage signal to the circuit IC1 proportional to the voltage of the capacitor C1.

Further, in the circuit of Fig. 1, a TM-Flyback power supply typically has a koi -.2 -.- 118025 coil transformer, through which coil N1 flows the current of a controlled switch and winding N3 supplies a voltage to the secondary side output.

The transformer still has a coil N2 which is used to monitor the transformer current magnitude. Coil N2 is further connected to circuit IC1.

5 A resistor R1 is connected to the current path of the switch component V1, above which the current of the switch component is measured as the applied voltage. This current information is further exported to IC1. Further, Fig. 1 shows the transfer of the output voltage information Uout from the secondary to the function controlling circuit IC1.

Such a prior art circuit shown in Figure 1 operates 10 such that the IC1 controls functions based on magnitude of supply voltage, output voltage, primary circuit current, and transformer current. The purpose is to provide a steady voltage to the output Uout. This is accomplished by comparing the magnitude of the current of the switch with a given current limit. The magnitude of the current limit is varied depending on both the output voltage and the supply voltage. This current limit is changed so that as the supply voltage drops, the current limit is also reduced and, as the load increases, the current limit is also increased at the output. The magnitude of the supply voltage is determined by the voltage divider switch 2 and the magnitude of the load directly influences the magnitude of the output voltage formed. Thus, such a power supply monitors the shape of the supply voltage curve and changes the current limit of the primary winding according to the supply voltage, thereby also correcting the power factor of the power supply. The primary winding current curve resembles • * ♦ ·· *; in this case, the shape of the supply voltage curve.

··. *: * Adjusting the current limit as described above causes problems; if the maximum output power of the device is set. For maximum output power monitoring,

* * -_L

i 25 lu is usually implemented in TM-Flyback connections, either by measuring the primary switch *: · or by measuring the load current in the secondary. This requires * * · ·.

often complex from a circuit solution before the data from the secondary has been transferred to the primary side or the current value of the switch component has been modified to a suitable standard: ·. si as a control signal.

! · · ·. 30 When the switch current exceeds a predetermined current limit and the control circuit • «instructs the user to switch off, the power is not switched off immediately, but only

• * I

: after transmission delay. In this case, the actual primary current exceeds the given * * * value. The magnitude of the crossing depends on the magnitude of the transmission delay, the supply voltage, the capacitance of the capacitor C1 and the inductance of the primary circuit. The current of the primary circuit, ·· *, 35 increases linearly, so that the increase in the supply voltage • * is directly proportional to the amount of primary current that exceeds the set current 3 118025. The transmission delay in the circuit is constant and is typically of the order of 50 to 200 ns. Thus, as the supply voltage changes with known TM-Flyback i power supplies, the primary current does not remain constant, which is a problem with power limitation.

5 Brief Description of the Invention

It is therefore an object of the invention to provide a method and apparatus implementing the method so that the above problems can be solved. The object of the invention is achieved by a method and a device characterized by what is stated in the independent claims. Preferred embodiments of the invention are disclosed in the dependent claims.

The invention is based on the fact that a simple coupling to the primary-side current limit is changed depending on the supply voltage, so that the primary current can be maintained at substantially the same syöttöjännitealu a very wide-range. This can be accomplished by using the 15 triple transformers used in the TM-Flyback connections to modify the current reference depending on the supply voltage.

An advantage of the method and apparatus according to the invention is the possibility of implementing output power limitation over a wide range of supply voltages by simple switching. Consequently, the overload information can be obtained directly as 20 isolated voltage information from the secondary and the circuit can be shut down in a simple manner without the commonly used foldback current limiting circuit. Further, the circuitry and method according to the invention allow the primary current limit ttml · to be set using a low resistance value in the primary circuit to produce a voltage signal. This significantly reduces the amount of losses in the power supply and thus improves the overall efficiency. In addition, the cost for the district will remain • * · *. ···. lower than before because a small resistor is enough to replace a high power resistor used earlier or even a current transformer.

♦ ♦: * ·· Brief Description of the Patterns • · »

The invention will now be described in more detail in connection with the preferred embodiments, with reference to the accompanying drawings, in which: ♦ ·. ···. Figure 1 shows the principle connection of a prior art TM-Flyback power supply; Figure 2 illustrates the functions associated with the invention of the integrated circuit used in an embodiment of the invention; and 4 118025 ί

Figure 3 shows the principle connection of an embodiment of the TM-Flyback power supply according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

Figure 2 shows the essential parts of an integrated circuit 5 for controlling TM-Flyback power supplies for the operation of the method and device according to the invention. The circuit is, for example, a type-labeled circuit of the L6561 of STMicroelectronics, designed specifically to control a high power TM-Flyback converter. It will be appreciated that although the L6561 circuit is used as an example in this specification, the apparatus and method of the invention may also be implemented using other commercial circuits or circuits manufactured from discrete components. i

As shown in Fig. 1, the input IC voltage to pin 3 is input to circuit IC1, such as L6561, j. Further, pin 5 provides current from transformer T1 auxiliary winding N2, pin 4 provides voltage to switch and transformer primer current, and pin 1 provides voltage information. Uout. Further, pin 7 of circuit IC1 is adapted to control a controllable semiconductor switch V1. Correspondingly, the numbered pins are shown in Fig. 2, which shows the internal structure of the circuit IC1 of Fig. 1 for purposes relevant to the invention.

The circuit shown in Fig. 2 acts as a controller for the operations of the TM-Flyback circuit as shown below. Pin 1 provides information on the magnitude of the output voltage. This information is suitably scaled so that it can be compared with the internal voltage of 2.5V in the IC1 circuit at the voltage regulator VC. The output of the voltage regulator is taken to a MULT unit which receives a voltage of 3 ... 25 don at its second input. The MULT unit provides these two information • · · ·. * ··. and thus provides a current reference to the comparator COMP, which compares the voltage information supplied from pin 4 from the measured current to the current reference.

... The output of the comparator COMP changes its state depending on which • ·· * ... higher current reference or the measured current value. Connected to this output is an SR flip-flop, • · **; · * 30, which produces the actual switch instruction, which is further amplified for the switch component by a control circuit DR, which supplies a pin 7 to control the semiconductor switch.

A zero current detection circuit ZCD, • · · M is connected to pin S of the SR flip-flop, which is supplied by pin 5, which in turn is connected to a transformer auxiliary winding • · · · · · · · · · · · · · · · When the auxiliary winding current goes to zero, all of the magnetic flux is lost from the transformer, whereby, according to the Transition Mode principle, the switch component is controlled. Upon sensing a drop in current to zero, the output of the ZCD circuit rises, and the output of the SR flip-flop also rises, and further, the control circuit DR directs the switch component to a conductive state. The component control, in turn, is terminated when the component current has increased beyond the current-5 limit, whereupon the SR flip-flop output changes its state controlled by the comparator circuit COMP.

Figure 3 illustrates in principle a circuit according to a preferred embodiment of the invention, based largely on the conventional TM-Flyback power source shown in Figure 1 and already described.

10, the transformer T1 winding N2 is connected via resistor R, diode D and zener diode Z to integrated circuit pin 3. Pin 3, which is the MULT input of circuit 101, also provides voltage information in reference to the reference voltage Vref, as shown in FIG. . The reference voltage is formed by a regulated voltage such as the pre-15 circuit operating voltage by means of a resistor 2. The purpose of this connection is to change the voltage of the MULT input in a manner according to the invention using the primary circuit current reference in response to the magnitude of the supply voltage. The circuit operates by pulling the MULT input of the IC1 circuit lower the higher the supply voltage. This is due to the fact that by the time the controllable switch-20 component reaches the conductive state, the end connected to the resistance R of the coil N2 becomes negative. Uni = (Uin-Uou.) XN2 / Ni * * · • · · · Jm: ': 25, where Uin is the magnitude of the voltage to be supplied, Uout is the magnitude of the output voltage · * ·, N2 is the number of winding turns of the winding N2 and N1 of the winding, respectively: ***; number of winding turns. The formula shows that the IC1 MULT input is drawn lower the higher the supply voltage. This, in turn, causes »». The current limit remains the same regardless of the supply voltage • ♦ · 30. The device according to the invention thus utilizes the pre-zero current measuring coils in the TM-flyback topology to set the current limit in response to the supply voltage.

According to a preferred embodiment of the invention, the power of the flyback -. · [·, When power supply is started is considered to be higher than the desired • · ♦ l /. ' 35 hours. This is accomplished in a coupling manner by connecting coil N2 to ** · * said MULT input via resistor R, diode D and Zener diode Z. The Ze-6 118025 ner diode prevents the current from passing through resistor R before starting the device, whereby the voltage of the MULT input of the IC1 circuit is initially slightly higher and accordingly the current limit. Such a procedure is important in the case of loads that require more power during the start-up phase.

5 The magnitude of the resistor R, together with the resistor switching circuit, determines how much the MULT input voltage is reduced as the supply voltage increases. Diode D, in turn, prevents the positive voltage of the end R of the coil N2 from being added to the MULT input when switch V1 is in the blocking state. With certain component choices, the use of a Zener diode Z is not necessary, but this results in an undesirable increase in the resistance R1. Similarly, if the zero-current winding N2 is also supplied with an operating voltage of n 15 V to the electrical circuit, its negative voltage according to the above formula is high, whereupon the resistor R must be very large if the MULT input is multi-volt.

It is advantageous for the operation of the device according to the invention that the supply voltage is as uniform as possible. This can be influenced in particular by selecting the filtering capacitor C1 as required.

The method and apparatus of the invention can limit the power of the secondary circuit without measuring the current in the secondary. Since the control mode as described above is such that the magnitude of the current remains constant with the load unchanged, the power limitation can be achieved by simply observing the secondary voltage. If the voltage of the secondary does not rise to the desired value but falls, there is overload in the secondary. Based on the voltage information, the device can be shut down in a simple manner by stopping modulation of the switch component.

Figure 1 or Figure 3 does not show in detail the output voltage output from the circuit to the primary. However, it is clear that the voltage can be determined by: ***; for example, by resistive voltage distribution, as in the supply coil.

* ··

It should be noted that the ♦ ·. the embodiment corresponds to the circuit described with reference to Fig. 1, except for the features which are characteristic of the invention. Furthermore, only those features which directly influence the operation of the solution of the invention are explained in the IC1 integrated circuit IC1.

It will be obvious to a person skilled in the art that the basic idea of the invention can be implemented in many different ways. The invention and its embodiments are thus not limited to the examples described above, but may vary within the scope of the claims.

Claims (11)

1. A method in connection with a Transition Mode Flyback power source, which method comprises steps in which a direct voltage is generated as a supply voltage for the power source, a primary circuit coupling component is controlled to a conducting state, the magnitude of an output voltage is determined, current limit of the primary circuit's current, the magnitude of the current in the power source's primary circuit is determined, and the primary circuit's coupling component is alternately controlled to a conducting state and a blocking state depending on the size of the primary circuit's current, characterized by the amount of the current's the current limit is formed in response to the sizes of the supply voltage and the output voltage, such that the current limit is reduced as the supply voltage increases, and increases as the supply voltage is reduced. 15 2. A method according to claim 1, characterized in that the formation of a current limit comprises steps in which a DC voltage is formed. the magnitude of the DC voltage changes in response to the size of the supply voltage and a current limit is formed on the basis of the changed DC voltage and the output voltage. **!:. Method according to Claim 1 or 2, characterized in that the method comprises the power source starting a step in which the current limit is kept at a predetermined time in a starting value greater than the maximum current limit. Method according to claim 1, 2 or 3, characterized in that the method further comprises steps in which it is detected that the output voltage is below the determined value, the control of the power source is terminated in response to the output of the output. j voltage becomes below the set value. : ***: 30 5. Transition Mode Flyback power source, which comprises ··· means for generating a direct voltage as a supply voltage for the power: .. f cold, • · * ·; · 'primary circuit controllable coupling component,: V: means for determining the size on an output voltage, means for forming a current limit of the primary circuit current, means for determining the magnitude of the current in the power source's primary circuit, and control means for controlling the primary circuit's coupling component in turn for a conductive state and a blocking state. condition in response to the magnitude of the current of the primary circuit and the current limit of the primary circuit, characterized in that the means for forming a current limit of the primary circuit current are arranged to decrease the current limit, where the supply voltage increases, and increase the current limit, where the supply voltage decreases. Power source according to claim 5, characterized in that with means for forming a current limit means means for forming a direct voltage, means for changing the size of the direct voltage in response to the size of the supply voltage and means for forming a current limit on the basis of the current voltage. changed the DC voltage and the output voltage. Power source according to Claim 6, characterized in that the means for forming a current limit for the primary circuit include a transformer (T1) secondary winding (N2) connected to the primary circuit, which is arranged to change the current limit depending on the supply voltage size to maintain the current limit. substantially constant. Power source according to any of the preceding claims 5, 6 or i44 *: · 7, characterized in that the means for forming a current limit for the primary ··· circuit current and the control means for controlling the switching component are in turn controlled: Ml ·: . *. 25 in a conductive state and a blocking state in response to the magnitude of the primary circuit current and the primary circuit current limit have been formed by an integrated circuit. 9. Power source according to any of the preceding claims 7 or 8, characterized in that the secondary winding (N2) of the transformer (T1) is connected to an integrated circuit, which determines a current limit for the primary circuit. J's current. IN*·*. Power source according to claim 9, characterized in that • · · ···. the transformer (T1) secondary circuit (N2) is connected to an integrated circuit via a resistor (R), a diode (D) and a Zener diode (Z), so that the cathode of the diode (D) is connected to the secondary circuit and the Zener the anode of the diode (Z) is connected to the customer circuit. 1 1 8025: 11 Power source according to any one of the preceding claims 5 - 10, characterized in that the secondary winding (N2) of the transformer (T1) is additionally connected to a detection circuit for the zero current of the integrated circuit. •; * * # • I • * · · * • · ···. * · ··· «• · · · · · · · · · · · · · · · · *** · • · * ♦ • · * · * ·« · · · · · * *… · • · · · · · · · · · · · · · A · · · · · a · · · · · · · · · · · · · ·